Measuring Op Amp Open-Loop Gain

[kingtutley]

It has been a very long time since I graduated and now work in a component dedication lab where I have been asked to verify the characteristics of an op amp. Most of this I can do without too much stress, but I have been unable to find a solution that makes sense to me for measuring open-loop gain.

As I understand it, open-loop gain is the gain of the op amp with input voltages (V+ and V-) equal to zero. The problem, as I understand it, is that the input offset is being amplified and possibly saturating the output.

So, if that's the case, could I not just measure the offset voltage (which is easy enough) then apply that voltage to the inputs in an open loop configuration such that the output voltage is zero (effectively nulling out the offset voltage), then increase the input voltage slightly (1-2 mV) and measure the output voltage again? Would this not give me the open loop gain?

It seems simple to me, but like I said, it's been a very long time since college, and I haven't done this kind of work since then.

Does anyone have any thoughts on this?

 

[Skogsgurra]

That is not a very common measurement. And, since the open loop gain is VERY frequency dependent, it is not possible to characterise gain over the frequency range you need.

Better then to close the loop using a 1 Mohm resistor feed-back and use an AC coupled input via a 1 kohm resistor - or thereabouts. Then apply a mV or probably uV AC signal that you adjust until the output barely clips. Read output and input signals and calculate gain. That method works also if you want to see the DC gain. But you have to remove the capacitor and also balance out the offset error (via a separate input and resistor) first.

I actually had to do such measurements only a few years ago. Old (around 1965) opamps built with discrete components were suspected and everyone thought they had gone bad in some way. They were not as good as today's zero drift opamps. But there was gain enough. Not 10^5 but around 10^4. And that was not any worse than they were in the data sheet. Some noise, yes. But no immediate need to rebuild the system. The problem was in the analogue set-point chain for the drives. Dust and dirt.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[IRstuff]

One issue with opening the loop is that you're now measuring the gain of a different circuit, because the voltages and currents no longer resemble that of the closed loop.

I remember something like that in EE114, Prof. Middlebrook came up with an approach. It's apparently called the Middlebrook method.

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[kingtutley]

I understand what you're both saying, but as I am being asked to essentially verify the datasheet, can I use a circuit other than an actual open loop circuit to prove open loop gain?

 

[BrianE22]

Analog.com had a nice write up:

https://www.analog.com/en/analog-dialogue/articles/simple-op-amp-measurements.html

 

[IRstuff]

The simple answer is no, since the datasheet value is only meaningful in a closed loop circuit, so verification of the datasheet value requires a feedback configuration as the other posters' link show in Figure 4.

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